I recently released a Drupal installation that has four language variations, each running off its own domain.

Within the localizer module configuration, the option exists to switch between languages by detecting the current hostname. Doing this comes with the clause:

If this option is active all the other switching options will be ignored.

This seems to be partially true. One crucial setting that wasn't being ignored was:

Redirect front page to the localized version

I had this option checked, assuming that it would be overridden by switching languages by hostname. Turns out, the setting wasn't being overridden. Whenever I tried to navigate to the root on any of my language variant domains, I kept getting redirected to the homepage of the default language.

It took me a while to track down what was going on. The module I was using is a couple of versions out of date now, maybe it's been fixed in the latest release.